1. Field of the Invention
This invention relates to a radiation image processing method, wherein a visible image which can be used for diagnostic purposes is produced from a read-out image signal. The read-out image signal is obtained from a read-out operation and is subjected to transformation processing. In the read-out operation, a stimulable phosphor sheet having a radiation image stored thereon is exposed to stimulating rays, which cause the stimulable phosphor sheet to emit light in proportion to the amount of energy stored thereon when it was exposed to radiation. The emitted light is photoelectrically detected, and thereby the read-out image signal is obtained.
2. Description of the Prior Art
When certain kinds of phosphors are exposed to radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays, or ultraviolet rays, they store part of the energy of the radiation. Then, when a phosphor, which has been exposed to the radiation, is exposed to stimulating rays such as visible light, it emits light in proportion to the amount of energy stored thereon during its exposure to the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor.
As disclosed in U.S. Pat. Nos. 4,258,264, 4,276,473, 4,315,318 and 4,387,428 and Japanese Unexamined Patent Publication No. 56(1981)-11395, it has been proposed to use stimulable phosphors in radiation image recording and reproducing systems. Specifically, a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet) is first exposed to radiation, which has passed through an object such as a human body, in order to cause a radiation image of the object to be stored thereon, and is then scanned with stimulating rays, such as a laser beam, which cause it to emit light in proportion to the amount of energy stored thereon during its exposure to the radiation. The light emitted by the stimulable phosphor sheet upon stimulation thereof is photoelectrically detected and converted into an electric image signal with a photomultiplier or the like, and the image signal is used to reproduce the radiation image of the object as a visible image on a recording material such as photographic film, on a display device such as a cathode ray tube (CRT), or the like.
In the aforesaid radiation image recording and reproducing systems, in order to eliminate various problems caused by variations in the input information and/or to obtain a radiation image, which has good image quality and can serve as an effective tool in, for example, the efficient and accurate diagnosis of an illness, it is desirable to ascertain the characteristics of the image input information before the radiation image is reproduced as a visible image so that the read-out gain can be adjusted to an appropriate value. The characteristics of the image input information depend on the conditions under which an image is recorded, such as the level of the radiation dose used when the image is recorded, on the image input pattern which is determined by what portion of an object (e.g. the chest or the abdomen of a human body) is recorded, and on the image recording method used, such as plain image recording or contrasted image recording. The process of determining the characteristics of the image input information will hereinafter simply be referred to as "ascertaining the image input information." It is also desirable to adjust the scale factor in accordance with the contrast in the image input pattern in order to optimize the resolution of the reproduced visible image. Also, in cases where image processing, such as gradation processing, is carried out on the read-out image signal, it is desirable that the conditions under which the image is processed be adjusted in accordance with the characteristics of the image input information.
The method as disclosed in U.S. Pat. No. 4,527,060 may be used to ascertain the image input information before the visible image is reproduced. In the disclosed method, a preliminary read-out operation (hereinafter referred to as the "preliminary readout") is carried out to ascertain the image input information of a radiation image stored on a stimulable phosphor sheet. A visible image which can be used for diagnostic purposes is obtained from a final read-out operation (hereinafter referred to as the "final readout"). During the preliminary readout, stimulating rays are used which have an energy level lower than the energy level of the stimulating rays used in the final readout. After the preliminary readout is completed, the final readout is carried out.
However, in cases where the preliminary readout is carried out, the processing of the preliminary readout signal obtained therefrom takes a long time.